Uniform response wide band amplifier



Jan. 13, 1942. o. H. SCHADE Fi led Oct. 26, 1939 TUUUTPUT 15 NETWORK lTOSUURCE 0/ W050 7 :5 3 vozmas *5; 32, 3 V I6 :5 T 8 INYENTOR. 077 0HSCHADE ATTORNEY.

Patented Jan. 13, 1942 UNIFORM RESPONSE WIDE BAND ANIPLIFIER Otto H.Schade, West Caldwell, N. J assignor to Radio Corporation of America, acorporation of Delaware Application October 26, 1939, Serial No. 301,220

1 Claim.

My present invention generally relates to wide band amplifiers, and moreparticularly to a novel method of, and means for, providing constantgain over the lower part of the frequency range of a wide bandamplifier.

Self-bias networks are highly desirable for use, in amplifiers havinghigh gm. Degenerative effects at zero frequency (D. C.) stabilize platecurrent and operating bias. This is particularly true in circuits of thetype disclosed by me in application Serial No. 211,155, filed June 1,1938, now Patent No. 2,243,442, May 2'7, 1941, wherein the self-biasingresistor is given a large value, and the grid resistor is returned to adirect current potential point which is positive with respect to ground,but is negative relative to cathode potential. If such an arrangement,however, is used for a wide band amplifier extending to low frequenciessuch as 60 cycles, the bypass condenser shunting the bias resistorcauses phase shift effects at the low end of the frequency range;decreased again at the said low end also occurs. Large values ofcapacity, of the order of 250 mf., have therefore been used intelevision video amplifiers to extend the low frequency range; in suchamplifiers the aforesaid problem was encountered.

One of the main objects of my present invention is to provide acompensation network in the output circuit of such a high gainamplifier, which network completely eliminates phase distortion or gainvariation down to zero frequency.

Another important object of my invention is to provide a high gainamplifier operating in the video frequency range and employing aself-bias network utilizing a bypass condenser; the amplifier outputcircuit utilizing a network to prevent gain reduction at the low end ofthe frequency range and permitting reduction of the value of the bypasscondenser capacity to a very low value.

Still other objects of this invention are to improve generally theefliciency and response characteristics of wide band, high gainamplifiers, and more especially to provide such amplifiers in a formsuch that they are reliable in operation and economical to manufactureand assemble.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims; the inventionitself, however, as to both its organization and method of operationwill best be understood by reference to the following description takenin connection with the drawing in which I have indicateddiagrammatically several circuit organizations whereby my invention maybe carried into effect.

"In the drawing- 1 Fig. 1 shows an amplifier stage embodying theinvention,

Fig; 2 illustrates two stages of a video amplifier embodying theinvention.

Referring now to the accompanying drawing, wherein like referencecharacters in the two figures designate similar circuit elements, thereis shown in Fig. 1 an amplifier tube l which may be a high gain pentodetube of the 1851 type. The tube is provided with a cathode 2, a signalgrid 3, screen grid 4, a suppressor grid 5 and a plate Gall in the ordernamed. The cathode 2 is connected to ground through a bias resistor l'oflarge value, the resistor being shunted by the bypass condenser 8. Acurrent source (D? C.) 9 has its negative terminal at ground potential,while its positive terminal is connected to the plate 8 through a pathincluding the load resistor! and compensation resistor l l arranged inseries. The condenser l2 shunts resistor H. The screen grid 4 isconnected to an intermediate point on source 9 to establish the screenat aproper positive potential. The signal grid 3 is connected, throughthe grid leak resistor l3 and lead I4, to a point on source 9 which ispositive relative to ground. The positive potential applied to grid 3reduces the excessively large negative bias developed across resistor lto the desired negative bias value. As previously ex plained in myaforesaid patent application, the use of the type of biasing arrangementdisclosed herein stabilizes plate current and operating bias, and isofespecial advantage in circuits using high gm tubes.

For example, where the amplifier is used to amplify video frequencies,say in a range up to 9 megacycles (mc.), the video voltage will beappliedbetween the input terminals marked Input, and the amplifiedvoltage will be taken from across the series resistors l0-I I. =Smallervalues. of condenser 8 cause phase shift and decreased gain at low videofrequencies; hence, large values of capacity (of the order of 250 mf.),are employed at 8 to extend the low end of the frequency range. Theinclusion of compensation network Il-I2 in series with load re,- sistorH] eliminates the aforesaid phase distortion and gain reduction, andmoreover permits reduction of the value of condenser 8 to amoderate'value, such as 1 mf. or less.

Without compensation the gain is substantially reduced at the lowfrequency end of the range,

because the input voltage must overcome the phase-opposed cathodevoltage developed across resistor 1, and the cathode voltage isproportional to the cathode transconductance 91110;) and the magnitudeR1 of resistor I. The network ||--l2 inserted in the plate circuit doesnot affect the gain at the high frequency end of the range, but zerofrequency gain Go increases to the value:

Im n( 1+ 2) m(k) l Where gmm) is the plate transconductance of amplifiertube l; R1 is the value of resistor l;

R2 is the value of resistor I I.

The value of R2 necessary for a zero frequency gain equal to the highfrequency gain is obtained from the following expression:

where I}: is the cathode current; Ib is the plate current.

In order to obtain further constant gain at other frequencies thecondenser I2 must produce the same phase shift of voltage acrossresistor H against voltage across resistor H], as the phase shift causedby condenser 8 of voltage across bias resistor 1 with respect to thegrid to cathode voltage of tube I. This is the case when the timeconstants are equal:

where C1 is the capacity magnitude of condenser 8, and C2 is thecapacity value of condenser [2. The correction network ll-l'2 is fullydetermined by Equations 2 and 3. The minimum value of capacities C1 andC2 depends on the shunt capacity Cs across the entire plate load (tubeand circuit capacities). C2 should be a short-circuit when theshunt-capacity C5 begins to aifect circuit performance. For a 1% errorthere is obtained C2=100 Cs.

By Way of specific illustration, and in no way restrictive or limiting,the following constants are given for an amplifier embodying'theinvention:

gm p =10,000 micro-mhos gm(k)=13,000 micro-mhos Ib= milli-amperes (ma.)

Ik=13 milli-amperes (ma) E (min.)=100 volts (v.) Eb=300 volts Resistor10=R1=3000 ohms Cs=20 micro-micro-farads (mmf.) Ec1(signal grid bias =-1volts Resistor 11=R2=17,000 ohms Resistor 7=R1=436 ohms Condenser12=C2=0.002 micro-farads (mf.) Condenser 8=C1=0.078 micro-farads (mf.)

A practical choice of values for the biasing and compensation networkscomprises assigning values of 0.1 mf. for C1 and 400 ohms for R1, whileC2 is made equal to 0.00256 mf., and R2 is given a magnitude of 15,600ohms.

In Fig. 2 there is shown the amplifier circuit of Fig. 1 embodied in amulti-stage video amplifier. The amplifier, in actual operation, had aflat frequency response from 20 cycles to 9 me. Here, as in Fig. l,amplifier tube I is of the 1851 type, and includes biasing network 18 inits grounded cathode lead. The signal grid 3 is connected to a couplingcondenser I5 which acts to transmit video voltage to the grid; the gridleak resistor l3 returns to ground through resistor l6, and condenser I1bypasses the junction of resistors l 3 and Hi to ground. Screen grid 4is connected to the positive terminal of the 300 volts current source 9through resistor 18, and bypass condenser l9 grounds the grid end of theresistor.

The junction of resistors I3 and ['6 is connected to the positiveterminal of current source 9 through resistor 20. The suppressor grid 5is connected to the positive terminal of current source 9 through aresistor 2|, while the grid end of resistor 2| is connected to groundthrough a resistor 22. The plate 6 of the amplifier tube is connected tothe positive terminal of current source 9 through a path which includescoil 30, coil 3|, resistor l0, resistor II and resistor 32 all arrangedin series. The junction of resistors II and 32 is connected to groundthrough the condenser 33, while the condenser I2 is connected to groundfrom the junction of resistors 10 and II. The succeeding amplifier 50 iscoupled to the junction of coils 30 and 3| through the condenser 40; thedesired negative bias is provided for the signal grid of amplifier tubethrough a path which includes resistor 5|. The plate shunt capacity Cpis shown in dotted lines asconnecting the plate end of coil 30 toground, whereas the reference character Cg2 denotes the dotted linecapacity connected between the junction of coils 30 and M to ground.

The network Il-|2 of Fig. 2 acts as the compensation network whichcorrects for the phase shift and reduced gain at the low frequency endof the video range caused by biasing network 'l-8. The compensationnetwork I l| 2 will not afiect the performance of additional networks toextend the high frequency end of the range, nor will it preventoperation of compensation circuits for correcting voltage losses andphase shifting due to the coupling condenser 40 and the leak resistor5|. In other words, there is shown in Fig. 2 in the plate circuit ofamplifier tube l various other compensation networks such as 32-33 and303l which act to compensate for other Voltage losses at low frequenciesand at the high frequency end of the video range. The following table ofconstants is given by way of illustration for the various elementsutilized in Fig. 2:

C15=0.05 mf. (317:0.004 mf. C8 =0.1 mf. C19=1 mf. C o cgzg l4 mmf.012:0.004 mf. 033:8 mf. 040:0.05 mf. R13=0.5 megohms R16=l0,000 ohms(variable between 5000 and 15,000 ohms. R2o=7,000,000 ohms R7 :400 ohmsR1a=75,000 ohms R21=200,000 ohms R22=10,000 ohms R32=3,0O0 ohmsR11=10,000 ohms R10=2,000 ohms R51=0.5 megohms L3o=60 microhenriesL3l=30 microhenries The resistor I S in Fig. 2 is adjustable in order tovary the signal grid bias, which in turn varies the values gim and gm(k)of pentode 1. This variation effects Equations 1 and 2 permittingtherefore, adjustment for perfect compensation in case the values R1,,R1 or R2 are somewhat in error as may be the case in practical circuits.

Resistors 20 and I6 provide a potential divider to obtain a smallpositive bias from source 9 to counteract the excess negative voltageproduced across element 1. Resistors 2| and 22 provide, also, apotential divider to obtain a small positive suppressor grid bias fromsource 9. The latter small bias causes the knee of the platecurrentplate voltage curves to occur at a lower plate voltage therebypermitting higher resistance values with consequently lower platevoltage in the plate circuit wthout loss of gm. The voltage drop acrossresistor I in Fig. 2 is in bucking relation to the voltage drop acrossbleeder "5. Assuming a drop of +5.2 v. across resistor I and a drop of+4.2 v. across bleeder I 5, then the effective bias of grid 3 will be -1v. Actually these values are not critical because of the degenerativeaction which causes the cathode current to readjust itself. The network32-33 compensates for phase shift and voltage drop at low frequenciescaused by coupling condenser 40 and resistor Coils 3| and 30 togetherwith tube capacitances Cp and Cg2 provide a low pass filter section, andthis network is operative at high frequencies only and extends thefrequency range, but does not interfere with the low frequencycompensation. In the absence of network I|-l2 the frequency responsewould drop at about 100,000 cycles to approximately 16% response at 1000cycles.

While I have indicated and described several systems for carrying myinvention into effect, it will be apparent to one skilled in the artthat my invention is by no means limited to the particular organizationshown and described, but that many modifications may be made withoutdeparting from the scope of my invention, as set forth in the appendedclaim.

What I claim is:

In a video amplifier network capable of amplifying frequencies in arange up to approximately 9 megacycles, an electron discharge tubeprovided with at least a cathode, a signal grid and an output electrode,a source of video voltage coupled between the signal grid and cathode aself-biasing network comprising a resistor of relatively high magnitudeof the order of 400 ohms located between said cathode and at a point ofrelatively fixed potential, means for applying the direct currentvoltage developed across said resistor to said signal grid, means forapplying a direct current potential of positive polarity to said signalgrid in opposition to said bias resistor voltage thereby to reduce thevoltage applied to the signal grid to a normally low negative biasvalue, a capacity of the order of 1.0 microfarad shunted across saidbias resistor, a pair of inductances and a load resistor seriallyconnected between the output electrode and cathode of said tube, acorrection network consisting of a resistor arranged in series with saidload resistor, a condenser shunting said correction resistor, saidcorrection network having its constants chosen to compensate for adecrease in response at the low frequency end of the video range, saidcorrection network and biasing network having substantially equal timeconstant values, and a second amplifier tube having its input electrodecoupled to the common terminal between said pair of inductances includedin the output circuit of the first tube.

O'I'IO H. SCHADE.

